Conveyors of this type are used where the slope of the conveying path upward or downward is so steep that the bulk material is no longer carried satisfactorily as a consequence of its properties and the excessively weak friction with the conveyor belt and it would slide downward. The practical possibilities of application include slope conveyors or elevators for loading silos, elevators for continuously operating ship unloaders and steep belt conveyors leading out of strip mining operations.
Cover belt conveyors comprise, in general, a horizontal or only slightly sloping section for picking up the material to be conveyed and a steep or vertical conveying section having a height difference for transporting the material to be conveyed. The material delivery may be located at the end of this steep or vertical conveying section or of a downstream horizontal or only slightly sloping section. The transitions between conveying sections of different slopes are made arc-shaped. Such a conveyor comprises, in general, a support frame, a basic belt and a cover belt as well as carrying and pressure rollers. The basic belt extends over the entire length of the conveying path, while the cover belt is needed only in the steep or vertically extending conveying sections, where the material being conveyed is to be pressed by the cover belt onto the basic belt and is to be carried by frictional connection. The bulk material compressed between the two belts forms a continuous conveying strand. Both the basic belt and the cover belt are driven or only the cover belt is driven, the cover belt being carried by frictional engagement in the latter case.
Due to the differences in the amount of material available for conveying, the conveying cross section must be able to be changed from zero to a maximum and the pressing pressure required in the particular case must always be present. A lower pressing pressure is sufficient in the case of an empty conveying strand as compared to that of a loaded one.
To overcome the height difference from the floor of the strip mine to the upper edge of the strip mine, a cover belt conveyor, in which the cover belt is pressed by rubber wheels against the basic belt, has been known from Durst/Vogt, Schaufelradbagger, pp. 111 and 112, 1986, Trans Tech Publications, Clausthal-Zellerfeld. Each of these wheels is articulated to the belt frame by a lever. These levers are directed obliquely against the cover belt in the conveying direction. The necessary pressing force is generated by springs. One set of these wheels is arranged in a row, so that the cover belt is pressed against the basic belt over its entire width. The drawback of this solution is that the wheels act on the cover belt in a narrow area only as a consequence of their circular tire cross section and the cover belt, which is flexible, in principle, and fits the surface of the material being conveyed, must therefore have a certain minimum transverse rigidity to distribute the pressing forces acting on the material being conveyed over the entire width of the flow of material being conveyed. Since it is impossible to meet both requirements on the bending strength of the cover belt at the same time, a compromise must be found in a mean value.
A cover belt conveyor, in which the cover belt is pressed onto the basic belt in one plane each by two rollers and a coil spring arranged between the rollers, has been known from DE-AS 15 56 581. The rollers are mounted on axle journals fastened pivotably to the frame of the conveyor and thus they press the edge areas. A reliable closing is thus guaranteed in the edge areas due to the special local introduction of forces and no material being conveyed can escape. The coil spring arranged in between, which rotates together with the outer rollers, has a somewhat smaller diameter than the two outer rollers and thus it preferably presses the middle area, which has a larger conveying cross section in the state of conveying. The characteristic of the coil spring may be selected to be such that the strongest pressing pressure is also reached at a large filling cross section.
The steep belt conveyor forming this class, in which this effect is achieved by pressing devices that are directed against the cover belt, has been known from DE 34 29 940 C2. These pressing devices comprise rotating arm supports, which are pivotably arranged on the frame of the conveyor and carry articulated pressure rollers at their free end. Torsion springs are provided to generate the necessary pressing pressure.
With a steep belt conveyor thus equipped, the cover belt can be pressed in the direction of the troughed basic belt with the volume of the material to be conveyed ranging from zero to the maximum and sufficient pressing can thus be applied to the material being conveyed. The pressure rollers can be adapted, in principle, to the filling cross section and the geometry and the material flow cross section due to the lever mechanism and the articulated mounting of the four loading rollers distributed over the entire width of the belt in the compensating brackets arranged as rocker systems. However, hollow spaces may be formed in the two outer areas of the troughed basic belt, because the pressure rollers press the cover belt onto the basic belt with their outer edges and the conveying cross section thus becomes smaller from the inside to the outside in a wedge-shaped pattern. The greatest wear between the pressure rollers and the cover belt also develops at these outer edges due to the local concentration of forces. The lever lengths of the rocker system are symmetrical, so that the force that is transmitted by the torsion spring to the support rollers arranged on both sides is distributed uniformly over the entire width of the belt. However, this is linked with the drawback that the pressure on the cover belt is the same over the entire width of the belt. However, a higher pressure is, in fact, needed in the middle area of the cross section of the bulk material as a consequence of the larger amount of material to be conveyed than in the two edge areas where a smaller amount of material is to be conveyed.
Another cover belt conveyor, in which the spring-loaded levers of the rocker systems with the pressure rollers are arranged on the belt frame above the cover belt on both sides relative to the belt width, has been known from DE 196 41 161 C2 (corresponding to U.S. Pat. No. 524,053; U.S. Pat. No. 524,056; U.S. Pat. No. 524,057; U.S. Pat. No. 524,058; U.S. Pat. No. 524,059; and U.S. Pat. No. 524,060). The longitudinal axes of the joints of the rocker systems extend in parallel to the direction of conveying. The lever lengths of the rockers carrying the pressure rollers are selected to be such that a stronger pressure is applied in the area in which the amount of material being conveyed is larger than in the areas in which the amount of material being conveyed is smaller. Due to the levers of the rocker systems being arranged on the belt frame at right angles to the direction of conveying, a transverse friction effect is generated due to the rolling resistance of the pressure rollers as a consequence of the yield of these levers due to the change in the axes of rotation of the pressure rollers in relation to the direction of conveying, as a result of which the entire dynamic behavior of the pressing behavior is adversely affected. An additional transverse friction effect is generated in this solution, as in the above-mentioned solution according to DE 34 29 940 C2, due to the fact that the distance between the cover belt and the mount of the pressing levers changes at different degrees of filling and a certain straight line on the circumference of a pressure roller performs a relative movement on the surface of the cover belt, whose direction is at right angles to the direction of conveying.
Furthermore, DE 41 38 226 C2 discloses a cover belt conveyor, in which the basic belt and/or the cover belt is/are provided with beads. The material to be conveyed can be received in a cross section thus shaped. The basic belt and the cover belt are pressed against one another by spring-loaded rollers. The beads are hollow and thus they can easily adapt themselves to the different amounts of material to be conveyed. An upper and lower support roller each, which are short and are mounted on one side, are arranged for guidance opposite each other in pairs at the bead-free outer edges on the return strand of the belt provided with beads. The cost of these conveyor belts provided with beads is considerably higher than that of simple, flat conveyor belts.